This invention relates to a scramble apparatus for distorting video signals and to a descrambler apparatus for restoring the same.
In order to feed specific video signals to only specific subscribers among the subscribers in a CATV system, it is necessary to transform the aforementioned video signal to an unreproducible condition for those subscribers other than the specific subscribers. For this purpose, means are utilized in which the specific video signals are distorted by the scrambler apparatus at the site of the transmitter during the transmission of the video signals, while the thus distorted video signals are restored into normal form by descrambler apparatus at the sites of specific subscribers.
It is known to provide scrambling and descrambling by what is called an AM gray sync system. That is, in the AM gray sync system, an arrangement is provided wherein at the transmitter, the relationship of the modulation levels between the horizontal synchronizing signal and the video signal in a RF stage is inverted so that the amplitude of the horizontal synchronizing signal is less than is previously experienced while the amplitude of the video signal is greater than is previously experienced. More specifically, a sine wave (an encode signal) of 15.75 KHz (or integral-fold thereof), which is in synchronism with the horizontal synchronous signal, is formed, by which the video signal undergoes AM modulation to thereby carry out scrambling; and to return this to is initial condition, a sine wave (a decode signal) of 15.75 KHz (or integral-fold thereof), of which the phase is reversed with respect to the firstly mentioned encode signal, is formed at the receiver, by which the scrambled video signal is restored to thereby carry out descrambling. In this case, however, there is a disadvantage such that the descrambled RF video signal is not completely identical to the unscrambled signal and includes a ripple. For example, let A cos .omega.t represent the RF video signal and B sin .theta.t the encode signal, then the scrambled signal is given by EQU (A+B sin .theta.t) cos.omega.t = A(1 + m sin .theta.t) cos.omega.t,
Where m = B/A. This is descrambled by the decode signal, which is reversed in phase with respect to the encode signal, then the resulting signal is given by EQU A(1 + m sin .theta.t) (1 - m sin .theta.t) cos.omega.t = A(1 - m.sup.2 sin.sup.2 .theta. t) cos.omega.t
indicating that the term of m.sup.2 sin.sup.2 .theta. t remains relative to the original signal A cos.omega.t. This ripple component is determined by the square of the modulation degree m, but preferably the modulation degree m is greater in terms of the object inherent in the scrambler. However, because the ripple component produced in the descrambled signal becomes greater as the modulation degree m increases, the result is that the reproduced image suffers from longitudinal bright and dark stripes, resulting in an extreme difficulty in viewing the image.